Shared classifieds?

I got an idea when looking at craigs list of classifieds. How about implementing a classified system in drupal, and have the the classifieds shared between all interested sites? A central place could be established, and everybody interested could aggregate the classifieds they want from that place. For instance, a computer site might like to have all used computers in a certain area listed, a drupal site might want all job listing containing drupal or php. These sites would also give the oppertunity to add classifieds. If this is a nice idea, many people would be better than me in explaining why :)